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Register a new userEvaluation of the $^{13}$N($alpha$,p)$^{16}$O thermonuclear reaction rate and its impact on the isotopic composition of supernova grains
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Nicolas de Sereville
Publication date
2020
fields
Physics
and research's language is
English
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It has been suggested that hydrogen ingestion into the helium shell of massive stars could lead to high $^{13}$C and $^{15}$N excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high $^{13}$C and $^{15}$N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context $^{13}$N($alpha$,p)$^{16}$O the reaction plays an important role since it is in competition with $^{13}$N $beta^+$-decay to $^{13}$C. The $^{13}$N($alpha$,p)$^{16}$O reaction rate used in stellar evolution calculations comes from the CF88 compilation with very scarce information on the origin of this rate. The goal of this work is to provide a recommended $^{13}$N($alpha$,p)$^{16}$O reaction rate, based on available experimental data. Unbound nuclear states in the $^{17}$F compound nucleus were studied using the spectroscopic information of the analog states in $^{17}$O nucleus that were measured at the Alto facility using the $^{13}$C($^7$Li,t)$^{17}$O alpha-transfer reaction, and spectroscopic factors were derived using a DWBA analysis. This spectroscopic information was used to calculate a recommended $^{13}$N($alpha$,p)$^{16}$O reaction rate with meaningful uncertainty using a Monte Carlo approach. The present $^{13}$N($alpha$,p)$^{16}$O reaction rate is found to be within a factor of two of the previous evaluation, with a typical uncertainty of a factor 2-3. The source of this uncertainty comes from the three resonances at $E_r^{c.m.} = 221$, 741 and 959 keV. This new error estimation translates to an overall uncertainty in the $^{13}$C production of a factor of 50. The main source of uncertainty on the re-evaluated $^{13}$N($alpha$,p)$^{16}$O reaction rate currently comes from the uncertain alpha-width of relevant $^{17}$F states.
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As suggested in a Comment by Peters, Phys. Rev. C {bf 96}, 029801 (2017), a correction is applied to the $^{13}$C($alpha$,n)$^{16}$O data of Harissopulos {it et al.}, Phys. Rev. C {bf 72}, 062801(R) (2005). The correction refers to the energy-dependent efficiency of the neutron detector and appears only above the ($alpha$,n$_1$) threshold of the $^{13}$C($alpha$,n)$^{16}$O reaction at about $E_alpha approx 5$ MeV. The corrected data are lower than the original data by almost a factor of two. The correction method is verified using recent neutron spectroscopy data and data from the reverse $^{16}$O(n,$alpha$)$^{13}$C reaction.
The thermonuclear $^{19}$F($p$,$alpha_0$)$^{16}$O reaction rate in a temperature region of 0.007--10 GK has been derived by re-evaluating the available experimental data, together with the low-energy theoretical $R$-matrix extrapolations. Our new rate deviates up to about 30% compared to the previous ones, although all rates are consistent within the uncertainties. At very low temperature (e.g. 0.01 GK) our reaction rate is about 20% smaller than the most recently published rate, because of a difference in the low energy extrapolated $S$-factor and a more accurate estimate of the reduced mass entering in the calculation of the reaction rate. At temperatures above $sim$1 GK, our rate is smaller, for instance, by about 20% around 1.75 GK, because we have re-evaluated in a meticulous way the previous data (Isoya et al., Nucl. Phys. 7, 116 (1958)). The present interpretation is supported by the direct experimental data. The uncertainties of the present evaluated rate are estimated to be about 20% in the temperature region below 0.2 GK, which are mainly caused by the lack of low-energy experimental data and the large uncertainties of the existing data. The asymptotic giant branch (AGB) star evolves at temperatures below 0.2 GK, where the $^{19}$F($p$,$alpha$)$^{16}$O reaction may play a very important role. However, the current accuracy of the reaction rate is insufficient to help to describe, in a careful way, for the fluorine overabundances phenomenon observed in AGB stars. Precise cross section (or $S$ factor) data in the low energy region are therefore mandatory for astrophysical nucleosynthesis studies.
The $^{12}text{C}(alpha,gamma){}^{16}text{O}$ reaction plays a central role in astrophysics, but its cross section at energies relevant for astrophysical applications is only poorly constrained by laboratory data. The reduced $alpha$ width, $gamma_{11}$, of the bound $1^-$ level in $^{16}$O is particularly important to determine the cross section. The magnitude of $gamma_{11}$ is determined via sub-Coulomb $alpha$-transfer reactions or the $beta$-delayed $alpha$ decay of $^{16}$N, but the latter approach is presently hampered by the lack of sufficiently precise data on the $beta$-decay branching ratios. Here we report improved branching ratios for the bound $1^-$ level [$b_{beta,11} = (5.02pm 0.10)times 10^{-2}$] and for $beta$-delayed $alpha$ emission [$b_{betaalpha} = (1.59pm 0.06)times 10^{-5}$]. Our value for $b_{betaalpha}$ is 33% larger than previously held, leading to a substantial increase in $gamma_{11}$. Our revised value for $gamma_{11}$ is in good agreement with the value obtained in $alpha$-transfer studies and the weighted average of the two gives a robust and precise determination of $gamma_{11}$, which provides significantly improved constraints on the $^{12}$C$(alpha,gamma)$ cross section in the energy range relevant to hydrostatic He burning.
The ratio between the rates of the reactions O-17(alpha,n)Ne-20 and O-17(alpha,gamma)Ne-21 determines whether O-16 is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by O-16(n,gamma) are recycled into the stellar environment. This ratio is of particular relevance to constrain the s process yields of fast rotating massive stars at low metallicity. Recent results on the (alpha,gamma) channel have made it necessary to measure the (alpha,n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars. We present a new measurement of the O-17(alpha, n) reaction using a moderating neutron detector. In addition, the (alpha, n_1) channel has been measured independently by observation of the characteristic 1633 keV gamma-transition in Ne-20. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (alpha,n) and (alpha, gamma) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature. A new O-17(alpha,n) reaction rate was deduced for the temperature range 0.1 GK to 10 GK. It was found that in He burning conditions the (alpha,gamma) channel is strong enough to compete with the neutron channel. This leads to a less efficient neutron recycling compared to a previous suggestion of a very weak (alpha,gamma) channel. S process calculations using our rates confirm that massive rotating stars do play a significant role in the production of elements up to Sr, but they strongly reduce the s process contribution to heavier elements.
The $^{14}$O($alpha$,$p$)$^{17}$F reaction is one of the key reactions involved in the breakout from the hot-CNO cycle to the rp-process in type I x-ray bursts (XRBs). The resonant properties in the compound nucleus $^{18}$Ne have been investigated through resonant elastic scattering of $^{17}$F+$p$. The radioactive $^{17}$F beam was separated by the CNS Radioactive Ion Beam separator (CRIB) and bombarded a thick H$_2$ gas target at 3.6 MeV/nucleon. The recoiling light particles were measured by three ${Delta}$E-E silicon telescopes at laboratory angles of $theta$$_{lab}$$approx$3$^circ$, 10$^circ$ and 18$^circ$, respectively. Five resonances at $E_{x}$=6.15, 6.28, 6.35, 6.85, and 7.05 MeV were observed in the excitation functions, and their spin-parities have been determined based on an $R$-matrix analysis. In particular, $J^{pi}$=1$^-$ was firmly assigned to the 6.15-MeV state which dominates the thermonuclear $^{14}$O($alpha$,$p$)$^{17}$F rate below 2 GK. As well, a possible new excited state in $^{18}$Ne was observed at $E_{x}$=6.85$pm$0.11 MeV with tentative $J$=0 assignment. This state could be the analog state of the 6.880 MeV (0$^{-}$) level in the mirror nucleus $^{18}$O, or a bandhead state (0$^+$) of the six-particle four-hole (6$p$-4$h$) band. A new thermonuclear $^{14}$O($alpha$,$p$)$^{17}$F rate has been determined, and the astrophysical impact of multiple recent rates has been examined using an XRB model. Contrary to previous expectations, we find only modest impact on predicted nuclear energy generation rates from using reaction rates differing by up to several orders of magnitude.
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